Typically, glucose can be the product of starch processing, which may be a two-step enzymatic process that catalyzes the breakdown of starch, involving liquefaction and saccharification. During liquefaction, insoluble granular starch is usually slurried in water, gelatinized with heat, and hydrolyzed by a thermostable alpha-amylase. The alpha amylases currently used in most commercial liquefaction processes are not stable at acidic levels of pH 4.8 to pH 5.2, and therefore the pH of the slurry is adjusted to about pH 5.6 to 6.0 using suitable alkali (e.g., sodium or calcium hydroxide, sodium carbonate or ammonia). Therefore, liquefaction is normally carried out at pH of about 5.6 to 6.0.
During saccharification, the soluble dextrins produced in liquefaction are further hydrolyzed by glucoamylases to produce sugars. Glucoamylases are exo-acting carbohydrases, capable of hydrolyzing both the linear and branched glucosidic linkages of starch (e.g., amylose and amylopectin). Commercially available glucoamylases typically have pH optimum in the acidic pH ranges (less than 5.0). As a result, pH of the liquefact is typically adjusted to acidic, in the range of about 4.2-4.5, using for example a diluted acid (e.g., sulfuric acid), to carry out saccharification. Therefore, currently liquefaction and saccharification processes for obtaining a desired end product, such as high density glucose syrups and high-glucose fermentation feedstock, are carried out at different pH levels. The fermentable sugars, e.g., low molecular sugars, such as glucose, may then be converted to fructose by other enzymes (e.g., glucose isomerases); crystallized; or used in fermentations to produce numerous end products (e.g., alcohols, monosodium glutamate, succinic acid, vitamins, amino acids, 1,3-propanediol, and lactic acid).
Because of the necessity to adjust pH levels after liquefaction and before saccharification, the currently available methods of processing starch-containing material require the use of extra chemicals, which increases production costs. The pH adjustments required after the liquefaction step to provide appropriate conditions for saccharification may also result in high salt accumulation in the reaction medium and a high sulphur content, creating an environmental disposal problem. Therefore, a need exists for a method of producing end products, such as high-density glucose syrups and high-glucose fermentation feedstock, commercially without having to carry out liquefaction and saccharification at different pH levels.